Problem: $ \left(\dfrac{5}{7}\right)^{-3}$
$= \left(\dfrac{7}{5}\right)^{3}$ $= \left(\dfrac{7}{5}\right)\cdot\left(\dfrac{7}{5}\right)\cdot \left(\dfrac{7}{5}\right)$ $= \dfrac{49}{25}\cdot\left(\dfrac{7}{5}\right)$ $= \dfrac{343}{125}$